POTASSIUM: WHAT IS IT?
According to Webster’s New World Dictionary, Second College Edition: A soft, silver-white, waxlike metallic chemical element that oxidizes rapidly when exposed to air; it occurs abundantly in nature in the form of its salts, which are used in fertilizers, glass, etc.: symbol, K; at. Wt., 39.102; at. No., 19; sp. Gr., 0.86; melt. Pt. 62.3°C; boil. Pt., 760°C. In the rest of this discussion, I’ll use the symbol K to stand for the word potassium.
Potassium is an alkali, with a single positive electrical charge.
WHAT DOES IT DO?
In the soil:
Being a positively charged, “base” element, K occupies a place in the Base Saturation calculation of a given soil. It is able to cling to negatively charged clay particles and displace weaker hydrogen ions from the clay colloid. Hydrogen is also a base and as such also occupies a place in the base saturation calculation as well. The presence of non-nutrient hydrogen in the soil accounts for soil acidity.
As a base element, K has a powerful influence on soil pH. Therefore, if the other bases (calcium, magnesium, sodium and certain trace elements) are low or absent, a soil high in K may still have a high pH reading. This is one reason why soil pH readings alone cannot be relied upon to make liming decisions. It is also a reason why growers should learn to understand the principles of cation exchange capacity and base saturation on their soil reports.
Excesses of K in the soil:
· Can kill seeds and seedlings. Too much manure can result in the buildup of excessive levels of K in the soil.
· Makes Ca and Mg unavailable to plants
· Excessive use of K fertilizer in the chloride form (Potassium Chloride/Muriate of Potash) may be detrimental to soil life.
· Calcium has a stronger positive electrical charge than K and can displace weaker K ions in the soil solution (one ion of calcium can push out two of K)¹.
In the plant:
· Vital to photosynthesis¹
· Improves water use effiency¹
· Increases yields¹
· Improves crop quality¹
· Reduces Disease¹
· Contributes to stalk strength/resistance to lodging²
· Contributes to winter hardiness²
· Contributes to protein, vitamin, enzyme, cellulose and carbohydrate production, sugar translocation, enzyme functions and cell division.²
· Catalyst for joining trace nutrients²
· “A 3% saturation of the exchange capacity is the usual eco-agriculture recommendation.”²
· Requires high microbial activity and correct amounts of soil boron to remain most available to plants.
Excesses of K in the plant:
· Reduce calcium and magnesium uptake from the soil.
· May contribute to the proliferation of pathogenic microbes(4)
· May encourage endophyte activity in fescue and rye(4)
· Will reduce uptake of sodium sufficient for animal health.
· Occur readily in grass plants
· Causes chloride levels to spike(4)
· Occur especially during sudden climatic changes.(4)
· Cool, wet conditions causes sodium, calcium and magnesium to decrease, while potassium spikes. Especially bad in prolonged droughts, followed by sudden changes after abundant rainfall, along with frosts and freezes; sudden heat elevations that cause a rapid grass growth in frost damaged plants(4).
· Promotes growth of pathogenic organisms.(4)
· May encourage excessive growth of endophyte and other pathogenic fungi, especially in fescue and rye.(4)
· Anything that suppresses the uptake of Calcium and Magnesium in the plant will result in the inability of the plant to biosynthesize protein, because this biosynthesis cannot take place without balanced levels of Ca and Mg. As a further consequence, the lowering of protein biosynthesis in the plant may contribute to excessive levels of carbohydrates and non-nutritive nitrogen.
Deficiency of K in the plant:
· Shows up in the oldest leaves (edges of entire leaf begin to die off, damage moves upward)¹
Additional Reading: “Soil, Grass and Cancer” by Andre Voisin, pp. 57-65 Specifically, potassium/magnesium disequilibrium, grass tetany
In the animal:
· Promotes normal growth and muscle function¹
· Cell regulating element²
· Regulates osmotic pressure in cellular tissue and fluids³
· Necessary for normal heart function(5)
· Found primarily within cells, whereas sodium is primarily found outside cells(5).
· Human requirement: Approximately 2,500 mg/day(5)
· Horse requirement: Approx. 1 mg/Kg dry matter intake.
· Sheep requirement: Approx. 0.8 mg/Kg max dry matter intake.
· Transmission of nerve impulses(5)
· Release of insulin from the pancreas(5)
Excesses of K in the animal:
· Interfere with correct calcium and magnesium metabolism: Depresses blood magnesium by interfering with magnesium absorption across the intestinal tract.
· Contributes to micronutrient (copper and selenium) imbalances(4)
· Contributes to grass tetany, milk fever, “downer cow” syndrome(4)
· Contributes to pathogenic microbe overgrowth in the gut(4)
· Contributes to immune suppression (4)
· Reduces the desire for sodium (salt) and water intake(4)
· Normal Na:K (salt to potassium) ratio in cattle saliva is greater than 10.0 (6)
· Are deadly to horses with the genetic disorder, HYPP (Hyperkalemic Periodic Paralysis).
Additional reading: “Soil, Grass and Cancer” by Andre Voisin, pp. 57-65 Specifically grass tetany.
POTASSIUM: HOW MUCH IS NECESSARY?
In the soil:
As a typical percent of the “base saturation” calculation on a soil test, K would be present in the soil at 2-5% with sodium at 0.5-3.0%. For the purpose of producing forage that provides correct nutrient levels for grazing animals, we propose a balance of K and Na at 2% of base saturation each, with Ca and Mg at optimal levels.
Depending on the goal of the producer, exactly where the K content of the soil falls in that range may be critical. If yield is the primary concern, it is likely that many soil consultants will prefer to keep the K level “topped up” at the 5% level or more. The amount of K that is required to support plant growth for the current season must be taken into consideration because there must be adequate K available for plant growth and nutrition. However, too much available K in the soil may be taken up by plants in amounts excessive for animal health, especially in the absence of adequate Na (sodium) and the other major cations, Ca (calcium) and Mg (magnesium).
Of all the cations, perhaps Na receives the least attention as a soil amendment because it is not required for plant health or yield. However it is taken up by plants and helps to balance the plant sap pH. Unless excessive sodium is a limiting factor in crop production, it will likely receive little attention on a soil audit. Like so many of the other minerals, it is generally taken for granted that sodium delivered to the animal in the form of supplemental salt will take care of animal nutrition requirements. Intake of supplemental salt varies widely from animal to animal and does nothing to correct the excessive K that may be in the forage as a result of cation imbalance in the soil.
From an animal health perspective, there’s more to “base saturation” than meets the eye. We would like to see optimal yield in our grass pastures, but not at the expense of animal health. As can be seen above, “luxury” amounts of K in the soil, especially if applied as fertilizer at the wrong time (especially in the form of potassium chloride), can account for deadly health issues in the animal.
At this time (2009) here at Blacklocust Farm (fondly known as Dogpatch), our aim is to drive the K down to about 2% base saturation in the soil, and to balance that with a sodium (Na) saturation of 2% while we continue to improve the balance of all other soil minerals. We do not know at this time (June, 2009) what the effect of this Na:K equilibrium will be, in terms of forage quality. The Fall, 2008 base saturation of K was 5.04%, slightly over maximum. The Na saturation was 0.99%, barely within the desirable range of 0.5-3%. A spring (4/26/09) fresh forage analysis revealed gross deficiency of sodium (in terms of animal health) in the grass, while the K was 2.55%, greatly in excess of animal requirements.
Potassium appears to be abundant in the parent rock of this region (Maritime Northwest). There will be an early 2010 application of trace mineral salt, but no K.
It is noteworthy that our horses do not voluntarily ingest any sort of loose or block supplemental salt. Excessive K in the ration diminishes the natural desire for sodium (salt). This has many indirect health hazards, some of which could be fatal.
In the plant:
I do not have specific information on how much K a plant actually needs. There are figures available for how much K a crop will remove from the soil for a given crop or season. However, we have research(7) that indicates the K level of alfalfa can be driven down to as low as 1% of dry matter without reducing yield or quality. Quoting: "It is particularly important to note that the alfalfa tended to accumulate K far beyond its critical need for the element...
"....the evidence strongly suggests that Ca, Mg, and K may each have not only a specific function that cannot be fulfilled by any other cation, but certain general functions of the type that can be performed by any one of these cations. Once the supply of any given cation has become adequate to meet its specific functional need, any additional quantity of it that may be absorbed by the plant is used only in these more general cation functions. To that extent, substitution of one cation for the other can be effected without detriment to the yield until a point is reached at which the content of the other is reduced below the critical value necessary for the specific functions it performs. The tendency of the plant to absorb K in excess of the amount needed to fulfill its specific function is greater than for either of the other two cations. (Calcium and Magnesium)”
Grasses are noted for their “luxury consumption” of K. Among other things, plants will uptake K not only for metabolic functions, but also to balance plant sap pH(8). If K is the most abundant and easily absorbed cation available, it will continue to uptake K to maintain its own pH.
In the animal:
· Sheep: 0.8% of dry matter intake.
· Horses: 1% of dry matter intake is thought to be the maximum tolerable level for HYPP horses
Horses are thought to “excrete excess potassium harmlessly” but in light of the above, the potassium itself is not the villain, it is the myriad metabolic problems indirectly caused by excess potassium that are of concern.
SOME PERSONAL OBSERVATIONS ON THE PERILS OF EXCESSIVE POTASSIUM
First of all, one has to define the word “excessive.” The required level of potassium for plants and animals is going to vary drastically from one source to another. For a university soil lab, concentrating on maximum yield, the term “excessive” will be one thing. For an Albrecht based consultant, it will be an amount that falls within the parameters of the base saturation for that soil, and will likely favor yield as well. For an animal nutritionist there may or may not be concern for “excessive” K if the animal “can excrete the excess harmlessly.” The nutritionist may simply suggest soaking the hay to remove excess K. For the owner of an HYPP quarter horse, “excessive” is over 1% of the daily dry matter intake.
It seems a reasonable goal to eliminate the work of soaking to remove both excessive sugars and potassium (and other soluble nutrients) when this practice can be eliminated by producing non-toxic feed in the first place.
As with so many human pursuits, when it comes to optimal animal health, the professionals and the producer are not always on the same page. The guy at the bottom, the animal owner, rarely has any idea there’s a problem until an actual health crisis occurs. The crisis can be the result of cumulative exposure over a number of years (such as ingesting high sugar grasses over a number of years resulting in EMS in horses), or it can be of seemingly sudden onset, as in grass tetany. In either case, the “cause” is usually blamed on a readily apparent stress riser, like “high sugar” in the grass, or “low magnesium” in the grass, when both health issues are really rooted in sick soil and metabolically corrupt grasses. Instead of correcting the fundamental problem, generally we are faced with a lifetime of questionable and time consuming management protocols such as soaking hay, or feeding magnesium blocks. Neither prevents the problem from happening again.
Why, for instance, would anyone tell you that winter colic or laminitis in horses could be due to excessive potassium? This is what I am suggesting. Look at the lists above. Excessive potassium in the forage reduces the craving for sodium in the diet. Salt and water intake are directly related. The animal reduces its intake of water and digestion is diminished at a time when it is on dry hay. Does impaction suggest itself? Excessive potassium fertilizes pathogenic microbes in the gut, releasing toxins and excessive ammonia. Does laminitis follow? Is offering warm water once a day the real solution?
Anyone with an EMS horse knows that magnesium is a key player in the complex. Excessive K drives down calcium and magnesium in the plant, and it interferes with magnesium and calcium metabolism in the body. And yet magnesium fertilizing of pastures is rarely suggested because it does not increase yield. No mention of animal health. Adding salt, calcium and magnesium to the feed except as an act of desperation doesn’t solve the underlying problem and may result in further unbalancing the animal’s metabolism. Supplementation doesn’t take care of the excess potassium in the grass that’s wrecking the magnesium and calcium balances in the first place. Doesn’t it make sense to get the “excessive” K OUT of the forage while building up levels of the other essential nutrients? It starts in the soil, not in the mineral box.
There is a “grass tetany” ratio presented by Woody Lane(9) that shows the relationship of Calcium, Magnesium and Potassium in the threat of grass tetany. A similar ratio appears in “Soil Grass and Cancer” by Andre Voisin. I ran a stack of hay analyses through the equation and the only one that turned up a threat for grass tetany was for an expensive hay that seemed to be the only predisposing factor in a case of winter laminitis. And on that report, it was the potassium that could have been considered excessive according to the grass tetany ratio. The calcium and magnesium levels were fine otherwise. Is there a link here? There was no other readily explainable disease-riser in the environment of the horse.
This represents the great flaw in the average hay analysis. All or most of the minerals can be comfortably within range according to National Research Council figures, but an excess of one mineral can in fact cause a “deficiency” or at least a disruption in metabolism so as to present a deficiency, of another.
What about negative personality impact? Does excessive K contribute to overexcitement of nerves in horses? We add Mg to the ration and sometimes that helps, but as often as not it doesn’t help nervous, excitable horses. Is it the excessive K, overstimulating the nerves and muscles? If that is the case, supplemental Mg used as a “tranquilizer” instead of a nutrient will probably not help, and could further upset the metabolism of the horse.
Supplementation of minerals to animals via the salt box is frequently unavoidable. But our dependence upon this form of animal nutrition is questionable because it misses addressing the foundation of animal health, or lack of it. You cannot rid the forage or the animal of excessive potassium by providing supplemental salt. Salt appears to be protective in the gut from the proliferation of pathogenic organisms(4), but an excessively high K:Na ratio in the animal can do its own harm¹. It does nothing to prevent the damage that seems to be done to magnesium metabolism. We try to offset magnesium deficiency by providing supplements, but that does nothing to forestall high K-induced pathogen proliferation in the gut, and we don’t really know whether the supplemental Mg is being absorbed, or if an excess of supplemental Mg is causing a metabolic imbalance of another kind.
Does excessive K contribute to laminitis in horses? EMS? The link between high K intake and grass tetany, downer cow syndrome and milk fever may not seem to suggest a direct relationship between high K intake, laminitis and EMS in the horse, but in light of the metabolic chaos that excessive dietary K seems to create, perhaps a wise owner sould consider the possibility.
How much is too much?
I have had a hard time pinning down just exactly what constitutes “excessive” for horses. Because 1% K of Dry Matter intake seems to be the maximum tolerable for HYPP (hyperkalemic periodic paralysis, a genetic disorder), that number sounds just fine to me for equines. I do not know whether that number is achievable without some sort of artificial diet. Grass plants seem to have a powerful addiction to luxury consumption of K. Don’t let the odd article that says legumes do not overindulge fool you. That statement is positively untrue. Firman Bear et. al. proved that alfalfa can get by just fine with a dry matter K content of 1%. I have seen a reference maximum of 3% K before forage enters the grass tetany danger zone, but I think 3% of dry matter is far, far too generous.
In my own forage analyses, with our high background K levels in the soil, and our recent entry into soil mineral balancing practices, the ratios of K and Na are reflected in K and Na ratios in the hay analyses; K is way over my theoretical limit, Na is grossly deficient.
Na (sodium) is apparently not required for plant health so from a crop yield point of view, Na only seems to be occupying space in the base saturation. It seems to be the orphan soil mineral, as copper is the orphan mineral for sheep in this country. With Na in the soil, generally all one reads about are the consequences of excessive salt. With sheep, one almost never encounters any useful information about deficiency, only the consequences of excess. Potassium, on the other hand, gives a big bang for the buck in yield and so is one of the Big Three, N-P-K. Large excesses of K can cause problems with crops. But how large applications of K for crop yield affect animal health does not seem to be a general focus of concern.
Na amendments to the soil will not improve yield, so unless an excess of sodium in the soil poses a crop risk, most consultants will ignore it in fertilizer recommendations. Consider these reasons to add sodium to the soil instead of the ration:
Adding salt to the ration is an accepted means of providing additional sodium to the animal, but it will not reduce the amount of excessive K the animal ingests. It may help to buffer some of the consequences of high K intake. But it does nothing to correct the suppression of Ca and Mg uptake by the plant. You can add Ca and Mg to the ration, but it seems to me that we’re entering a runaway downward spiral of patching up degraded animal health with ground rocks. If the correct level of nutrients is achieved in the soil so as to solve the excessive K problem in the first place, thereby supplying adequate Ca, Mg and sodium in their most available forms – through the plant – the whole house of cards that we call mineral supplementation is shown to be the shaky structure that it is.
When we were first investigating the mysterious deaths of our sheep – which turned out to be a copper deficiency – we had two lamb’s livers analyzed for mineral content. At the time we were only looking at copper. Having recently become interested in the possible link of excessive potassium to metabolic disorders in the animals, I pulled those liver analysis reports out and reviewed them again. Sure enough, both livers, one from 2005 and one from 2006, were red-flagged by the lab as being excessively high in K. On one report, the chemist made a notation that the significance of the elevation was not clear. While the magnesium level of the liver was good, the calcium level was very low, almost below the radar. This alarmingly low calcium level caused not one professional to raise an eyebrow.
I can draw no conclusion other than the high K was interfering with the calcium level because there is no reason to suspect that the Ca level in the ration was deficient. All forage analyses indicated adequate levels of Ca – not high, but within NRC parameters.
Could the high level of K have caused a borderline deficiency of Ca? I don’t know. The liver isn’t talking. It just suggests that excessive levels of minerals such as K can cause secondary deficiencies of other minerals, even when the ration meets NRC requirements.
This was partly the case with our copper deficiency problems. The Cu was deficient to begin with, but combined with high levels of molybdenum and iron, the Cu deficiency became deadly. But ask any American sheep producer or animal nutritionist and they will say that feeding copper to sheep is the equivalent of being an ax murderer.
As far as the sodium content of the liver, measuring tissue levels appears to be an unreliable means of measuring body status. There was no reference range for Na on the reports. I have reason to suspect it was low. If it was, it is obvious that the free choice supplementation of loose salt was not adequate to overcome the gross deficiency in the forage. .
I am not here to draw conclusions as to the effect of excessive potassium in the ration of horses and ruminants, but I’d be lying if I said I didn’t have some opinions. This is just a gathering of possibilities. For horse folks who soak their hay to remove excess carbs and potassium, it is not becoming apparent that the damage has already been done before the grass was ever cut for hay? Is a lifetime of soaking hay to remove excess K, along with other soluble nutrients, really the answer, or is it just another band-aid, a task made necessary by agricultural practices that value yield over animal health? If it becomes necessary to soak the hay for the current horse, what is the fate of the next foal? If we always do what we always did, we always get what we always got. Or, as they say, doing the same thing over and over and expecting a different result is a form of insanity. More sick animals. More wrong answers. More band-aids. More unnecessary animal suffering, wasted money and time managing symptoms.
Is excess K the stealthy bandit stealing our animals’ health? I don’t know. But I aim to find out.
Strategies to avoid excess potassium in forages
· Learn to understand the importance of the Cation Exchange Capacity (sometimes called the Total Exchange Capacity), and Base Saturation on your soil report. For this you must consult handbooks that base their explanations on the work of William A. Albrecht and others. I recommend obtaining a copy of The Ideal Soil by Michael Astera, www.soilminerals.com. Michael’s approach to soil fertility has as much or more to do with maximum nutrition as it does yield. See below for further reading recommendations.
· Test your soil! Do it now, so you can be ready with future fertility strategies. With an understanding of CEC and Base Saturation you can now look at your potassium level and decide for yourself if there is too much K. Work with an Albrecht based consultant who is sensitive to the impact of soil minerals on animal health. Again, I recommend Michael Astera, www.soilminerals.com. Presently, it appears that 2% base saturation of K and 2% base saturation of Na (sodium), along with optimal levels of Ca and Mg are desirable to keep K on a leash in forages, and assure that nutritionally optimal levels of the critical macro minerals, Calcium, Magnesium and Sodium available. Sodium is an “orphan” element that is often ignored as a soil amendment. 1% minimum in forage is required for animal health. Excess K and deficient Na will result in unhealthy levels of both in forage and hay.
· If your soil test reveals deficiencies and/or excesses of any of the bases, you can be assured of nutritionally unbalanced forage. Mineral supplementation to animals will not entirely solve the potential health hazards of fundamentally “toxic” forage.
· Learn how much supplemental K it takes to feed the crop for the year without increasing soil K more than to about 2% of base saturation and then plan future applications of K accordingly.
· Don’t apply potassium fertilizers in the spring.
· Don’t use muriate of potash! It is potassium chloride, which may be toxic to soil microbial life. Potassium sulphate is more microbe-friendly and can add needed sulphur to soil. The sulphur helps protein synthesis from all those extra carbs.
· Consider using salt as a pasture amendment if the base saturation of Na is below 2%. See references below for more information on fertilizing pastures with salt. The extra sodium will help displace some of the K in the forage. Salt fertilization will cause Na levels in forage to peak at about 6 weeks after application, and thereafter decline for a total of about 30 weeks, but will leave higher levels of Na in the soil than previously. Time your salt application so that Na availability will peak when animals are turned out onto grass in early spring.
· Consider using a biostimulant such as molasses on early spring pastures to encourage microbes to move more magnesium up into the plants. The forages need all the help they can get to move Mg into the plant in cold soils. K is much more active at these times.
· Test your fresh forages at critical times of year! This includes cool, damp weather conditions in early spring, where magnesium uptake is naturally low and potassium is rocketing into the plants. Also, frost damaged early spring grass is at risk for potassium and chloride spikes. (Do not neglect to call for chloride levels in both soil and forage tests. Also ask for nitrate levels, which are separate from the N in crude protein. Over 1% Nitrate is considered dangerous). Also, be careful of grasses growing rapidly after droughts break, and with spikes in temperature.
· Test your hay. Observe whether K levels are over about 1.5%. Compare to the Na content of the grass. If the K is excessive and the Na is deficient, do not expect supplemental salt to handle all the potential damage that excess K can do. You may also see lower than desirable levels of Ca and Mg on the hay report. This is because the excess K suppresses the uptake by plants of calcium, magnesium and sodium. Even if the levels of Ca, Mg and Na fall within NRC parameters for animal health, excessive K can still create a deficiency in the body by interfering with metabolism. (See above re testing for chloride and nitrates.)
· For horses, be aware that excessive carbs in grasses are also a result of unbalanced soil minerals. This imbalance can also lead to excessive nitrogen in grasses. Manage your horses’ grazing to avoid seasonal spikes in K and low Mg, but be aware that grazing management will not be optimal until the soil chemistry is right. You can’t control the weather, but you can control the soil minerals to a great degree.
· If you are forced to buy hay or pasture your horse on someone else’s land, consider negotiating to take control of your forage source. With an understanding of CEC and Base Saturation, and a good consultant (university soil labs are not usually interested in animal nutrition, only yield) you have much more control over your animals’ health.. Investigate the possibility of leasing a certain amount of land with the option to correct the soil chemistry. Form a co-op with other horse owners. Try to think of a way to become proactive to gain access to the soil that feeds your animals.
· Fundamental animal health without band-aid type symptom suppression begins in the soil. Question whether 3% K in forage is really acceptable, especially in light of the fact that the maximum requirement for a sheep is 0.8%, and over 1% is too much for an HYPP horse. Question whether sodium levels on a hay analysis below life sustaining levels are acceptable.
· Question whether mineral supplementation as the norm rather than the exception will result in complete body-building, immune-supporting, growth, vitality, reproductive health.
Glossary:
Anion: Negatively charged ion. See Ion. The major anions in soil are Nitrogen, Phosphorus and Sulphur
Base: A substance which forms a salt when it reacts with an acid; in terms of the modern theory of acids and bases, a substance that removes hydrogen ions (protons) from an acid and combines with them in a chemical reaction³.
Base Saturation: Percentages of the Cation Exchange Capacity that are occupied by the various “bases” or positively charged minerals. The desirable base saturation differs for individual soils, and is based on the Cation Exchange Capacity of that soil.
Cation: Positively charged ion. See Ion. The major cations in soil are Calcium, Magnesium, Potassium, Sodium and Hydrogen
Cation Exchange Capacity: A measure of the soil’s ability to attract and hold cations, minerals with a positive electrical charge.
Colloid: Smallest possible particle of clay or humus.
Ion: An electrically charged atom or group of atoms, the electrical charge of which results when a neutral atom or group of atoms loses or gains one or more electrons during chemical reactions, by the action of certain forms of radiant energy, etc; the loss of electrons results in a positively charged ion (cation), the gain of electrons in a negatively charged ion (anion)
Potassium: symbol, K; at. Wt., 39.102; at. No., 19; sp. Gr., 0.86; melt. Pt. 62.3°C; boil. Pt., 760°C. Potassium is an alkali, with a single positive electrical charge.
References:
¹Hand-On Agronomy, Neal Kinsey
²The Biological Farmer, Gary F. Zimmer
³Webster’s New World Dictionary, Second College Edition
(4)Don’t Short Salt, T. W. Swerczek, DVM, Ph.D for www.beefmagazine.com
(5)Conversations on Chelation and Mineral Nutrition, H. DeWayne Ashmead, Ph.D., F.A.C.N.
(6)Use of Salt in New Zealand Pastoral Farming, Dominion Salt www.dominionsalt.co.nz/acatalog/SaltinNZpastoral.pdf
(7)Firman Bear et. Al.
(8)Michael Astera, www.soilminerals.com
(9) Too Little, Too Much, Early Spring” Woody Lane
http://www.tein.net/~msufergus/Ag/livestock/Grass%20Tetany%20Article.pdf
Further Reading:
The Ideal Soil Handbook: Michael Astera, www.soilminerals.com
Soil, Grass and Cancer: Andre Voisin
Hands On Agronomy: Neal Kinsey
Eco-Farm, An Acres USA Primer: Charles Walters
The Biological Farmer: Gary F. Zimmer
